When EndNote is 1st launched you may be asked to create a default library. Libraries are the EndNote databases that store references, and associated files. You can have one default library for all references, or create multiple libraries.
To create a new EndNote library go to File > New and then name the library.
EndNote is very good about automatically saving references every time you add or edit one to your library, but to make a copy of an EndNote library to use on another computer, or to send to a colleague create a compressed copy of the library. Using the the save a copy option is not recommended when moving libraries between computers or attaching to email.
Go to File > Compressed Library (.enlx)...
Select the appropriate options and click Next and then Save.
Groups are subsets of references. Use the Groups drop down menu, and select "Create Group" to create a new group.
Highlight references you wish add to a group.
Either right click or go to "Add References To" and select the group to copy the selected references to. You can also drop and drag references to a group.
Change where and what panels (Preview, PDF viewer, and Groups are displayed on your screen by clicking the layout button located in the lower right.